1. Field of the Invention
The present invention relates to a method of producing D-psicose using a D-psicose epimerase (hereinafter, referred to as psicose 3-epimerase) derived from Agrobacterium tumefaciens. 
2. Description of the Related Art
In general, D-psicose is an epimer of D-fructose, and is very similar to D-fructose in the aspects of intensity and type of sweetness. However, unlike D-fructose, D-psicose is hardly metabolized during assimilation in the body, and has a low degree of caloric contribution. Thus, D-psicose can be used as an effective ingredient of diet foods. While sugar alcohols, which are widely used as a non-sugar sweetener, cause side effects such as diarrhea when more than a stipulated dose is ingested, D-psicose has no such side effects. Furthermore, D-psicose has a calorie value of nearly zero, as it is almost non-metabolizable in the human body, and has a function of reducing abdominal fats by suppressing the activity of lipogenic enzymes. Therefore, D-psicose can be used as a sweetener which is also helpful in weight reduction (See, for example, Matsuo, T., Y. Baba, M. Hashiguchi, K. Takeshita, K. Izumori and H. Suzuki, “Dietary D-psicose, a C-3 epimer of D-fructose, suppresses the activity of hepatic lipogenic enzymes in rats,” Asia Pac. J. Clin. Nutr., 10:233-237 (2001); and Matsuo, T. and K. Izumori, “D-Psicose, a rare sugar that provides no energy and additionally beneficial effects for clinical nutrition,” Asia Pac. J. Clin. Nutr., 13:S127 (2004)).
In this light, D-psicose is attracting great interest as a dietary sweetener, and there is an increasing demand in the food industry, for the development of a method of producing D-psicose efficiently. This is because D-psicose only exists in a very small amount in nature as an intermediate during the process of theriae treatment or glucose isomerization reaction, and cannot be chemically synthesized.
Therefore, in order to address such problems as described above, research is being conducted on methods of enzymatically producing D-psicose using D-fructose as a substrate.
However, there is a problem in the methods developed so far, that the production costs are high, owing to the low yield of D-psicose.